Mark,
The choice between 308Win and 30-06Spg largely depends on what you are going to use for the nib.
If you are going to use a slimline kit nib, then the 308Win is easier. The shellcase is about the same length as a slimline tube, and everything fits just like a normal slimline. With 30-06, you'd have to use a longer tube (such as a euro lower tube).
If you are going to use a real (drilled-out) bullet, then 30-06 is typically easier. With an appropriate bullet seated at the right depth, the slimline tube fits nicely in a 30-06Spg case. The transmission seats a little deeper than a regular slimline pen, but it still works fine. If you seat a bullet in a 308Win case to factory dimensions, you wind up with a lower barrel that is not long enough to get a good grip on the transmission. Some people seat the bullet extra long to make up for that, but I think that makes an unnatural looking cartridge. Instead, I make 308Win pens with a tube that sticks out of the case head about 3/16". The upper tube then needs to be recessed by a corresponding amount.
I hope that helps,
Eric